1. Field of the Invention
This invention relates to dipole based decoy systems for protecting air vehicles against radar directed weapons and terminal defense systems.
2. Description of the Related Art
Air vehicles including fighter jets, unmanned drones, strategic and tactical missiles and artillery shells are susceptible to engagement by radar directed weapons such as guns, surface-to-air missiles (SAMS) or terminal reactively launched explosives. These defensive weapons systems pose a serious danger to pilots, survivability of the offensive weapons and the efficacy of the mission. As radar defenses become more sophisticated to engage and defeat traditional countermeasures, the air vehicle anti-defense systems must adapt.
During World War II, it was discovered that radar could be confused by the use of strips of aluminum cut into lengths representing the half wavelength of the radar frequency threatening the air vehicle, e.g. a “dipole”. This invention was called “CHAFF” and is still used extensively by all air forces in combat. More recent developments in chaff technology include the use of aluminum-coated glass filament and silver-coated nylon filament.
Tens to hundreds of thousands of these strips may be packaged into a dispenser and dispersed as necessary to present false target information to confuse the enemy. Chaff is typically packaged in units about twice the size of a cigarette pack. When individual fibers of such a unit are widely dispersed in the atmosphere they create a radar echo similar to that of a small air vehicle or missile. If a stronger echo is wanted, one dispenses two or three units simultaneously.
The effects produced by chaff depend upon the manner in which it is used. If the bundles are dropped continuously they will cause a long line of radar returns across a radar scope. Several side by side stream drops will form a chaff corridor and an air vehicle flying within that corridor cannot be seen by certain radars using certain frequencies. These applications of chaff constitute a form of jamming.
Chaff bundles may also be dropped randomly in which case the radar scope may become filled with chaff returns so that the radar operator has difficulty finding the air vehicle. This is a deception technique similar to false target generation. Finally, chaff may be dropped in bursts of several bundles. Against tracking radar, a chaff burst will create a larger radar echo than the dropping vehicle. Thus, the radar will tend to lock on to the chaff rather than the air vehicle.
One problem that all forms of chaff have is that, once dispensed, the chaff immediately decelerates and floats to the ground while the air vehicle dispensing it continues on its flight path, leaving the protection of the chaff. Additionally, radars using Doppler gating can reject chaff due to low velocity and reacquire the air vehicle. Radar may also reacquire the air vehicle by using a moving range gate. Consequently, to defeat the more sophisticated radar defense systems air vehicles must rely on expensive active jammers, expensive stealth treatments, or very low terrain following tactics to augment the deployment of chaff.
For high end air vehicles such as fighter jets and strategic missiles, a combination of chaff, active jamming, stealth technology and low terrain guidance is a viable although sub-optimal solution. However, as radar defense systems and, in particular, terminal defense systems at the target become more sophisticated and more prevalent it is becoming apparent that low end air vehicles such as tactical missiles, drones and artillery shells must also be protected. These weapons systems cannot support the expense associated with current countermeasures. Thus, there remains an acute need for an alternative to chaff that cannot be overcome by Doppler gating and is compact, lightweight, reliable and inexpensive.